I am not a fan of fictional incest myself, but I find the reasoning behind their relationships really interesting. Utau never really had her father in her life. She lacked any male affection up until Ikuto started paying attention to her. He was the person who raised Utau, who helped shape her into who she is today. That led to Utau growing fond of Ikuto, and thus explains her determination to give him the Embryo and help him in any way that she can. Utau's affection towards her brother is not just there for the fun of having an incest ship. Utau loves Ikuto because he gave up his childhood to raise her. He was the only person who ever cared about her and gave her the time of day. Utau loves Ikuto because she was lonely and he was always there.

I actually think it's kind of cute. (I'm sorry) I mean, yeah, incest can get weird and everything but, Ikuto is the only one she could rely on. The only one that actually cared and loved her. So of course she will love the only person who cared for her when she needed it most